The patterning of radiation sensitive polymeric films with high energy radiation such as photons, electrons, or ion beams is the principle means of defining high resolution circuitry found in semiconductor devices. The radiation sensitive films, often referred to as photoresists regardless of the radiation source, generally consist of multicomponent formulations that are coated onto a desired substrate such as a silicon wafer. The radiation is most commonly ultraviolet light at wavelengths of 436, 365, 257, 248, 193 or 157 nanometers (nm), or a beam of electrons or ions, or ‘soft’ x-ray radiation, also referred to as extreme ultraviolet (EUV) or x-rays. The radiation is exposed patternwise and induces a chemical transformation that renders the solubility of the exposed regions of the films different from that of the unexposed areas when the films are treated with an appropriate developer, usually a dilute, basic aqueous solution, such as aqueous tetramethylammonium hydroxide (TMAH).
Typical photoresists contain a polymeric component and are generally comprised of a polymeric matrix, a radiation sensitive component, a casting solvent, and other performance enhancing additives. The highest performing photoresists in terms of sensitivity to radiation and resolution capability are “chemically-amplified” photoresists, allowing high resolution, high contrast and high sensitivity that are not generally provided by other photoresists. Chemically amplified photoresists are based on a catalytic mechanism that allows a relatively large number of chemical events such as, for example, deprotection reactions in the case of positive photoresists or crosslinking reactions in the case of negative tone photoresists, to be brought about by the application of a relatively low dose of radiation that induces formation of the catalyst, often a strong acid.
Although chemically-amplified resists have been developed for 248, 193 and 157 nm lithography, certain barriers to achieving higher resolution and smaller feature sizes remain due to physical, processing and material limitations. One such phenomenon that arises for imaging in the sub-50 nm regime, resulting in diminished image integrity in the pattern, is referred to as “image blur” (see, e.g., Hinsberg et al., Proc. SPIE, (2000), 3999, 148 and Houle et al., J. Vac. Sci. Technol B, (2000), 18, 1874). Image blur is generally thought to result from two contributing factors: gradient-driven acid diffusion and reaction propagation, the result being a distortion in the developable image compared to the projected aerial image transferred onto the film. Although, both factors contribute to image blur, the degree of the effect from each is different. Temperature also has a differing effect on each factor.
Acid diffusion is further thought to depend on several factors, including the type of photoacid generator (PAG) and the mobility in the photoresist polymer. In turn, the acid mobility in the polymer is dependent on a variety of factors, including, among others, the chemical functionality of the polymer, and the temperature and time of baking during resist processing.
Reaction propagation likewise depends on a number of factors, such as the activation energy (enthalpy) and the volatility of products (entropy).
Thus, as the need for better resolutions, minimum feature sizes, improved sensitivity and process latitude increases, image blur due to both contributing factors must be mininized. While both may be reduced to a degree by the use of acid-quenchers, or bases, the extent of thermally induced image blur estimated to be on the order of 10–50 nm with conventional resists and processing (see also Breyta et al., U.S. Pat. No. 6,227,546) suggests that improvements are necessary in order to achieve sub-50 nm imaging.
An ongoing need therefore exists for new photoresist materials and compositions, as well as methods of patterning substrates, which can lead to improved high resolution photoresist applications.